METALLURGICAL JOINING OF STEEL SHEETS TREATED BY NITROOXIDATION BY A HYBRID CMT LASER PROCESS

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1 METALLURGICAL JOINING OF STEEL SHEETS TREATED BY NITROOXIDATION BY A HYBRID CMT LASER PROCESS Ivan MICHALEC, Jozef BÁRTA, Jaroslav JAN ÁR, Katarína BÁRTOVÁ, Milan MARÔNEK Slovak University of Technology, Faculty of Materials Science and Technology, Department of Welding, J. Bottu 25, Trnava, Slovak Republic ivan.michalec@stuba.sk, jozef.barta@stuba.sk, jaroslav.jancar@stuba.sk,katarina.bartova@stuba.sk, milan.maronek@stuba.sk Abstract Nitrooxidation belongs to nonconventional technologies of steel sheets surface treatment. Material properties like corrosion resistance and mechanical properties can be improved by nitrooxidation process. Article deals with the possibility of joining this way treated thin steel sheets by the hybrid process of laser CMT brazing. Low-carbon deep-drawing steel DC 01 with the thickness of 1 mm was used as a base material for nitrooxidation and solder CuSi3 was used as filler in brazing process. Total amount of 15 specimens with different process parameters were produced in the First Welding company, Inc. in Bratislava. Basic defects like irregular and reinforced bead were observed during visual inspection. Specimens that passed through the visual inspection were thereafter examined. Macroscopic analysis proved the presence of extensive porosity in full length and cross-section of the joint. Presence of coarse-grain area which was mainly caused by the heat input from hybrid brazing process was observed during the microanalysis. Microhardness measurements showed high solder hardness and therefore the further chemical exanimation was focused on interface of solder HAZ. Due to the laser preheat the chemical composition of solder was changed in process of hybrid brazing. Tensile strength test confirmed previous results, as the crack appeared in solder in all tested specimens. Technology of hybrid laser CMT brazing was evaluated as unsuitable for joining nitrooxidised thin steel sheets, regarding the previously mentioned irregularities of the bead and extensive porosity which is not permitted in manufacture. Keywords: nitrooxidation, metallurgical joining, cold metal transfer 1 INTRODUCTION Steel sheets with surface treatment are widely used in an automotive industry which works as motive power for their research, design and production. There are also investigated new methods of surface treatment having major influence on improvement of surface properties of steel sheets while keeping the price at favourable level. One of the suitable surface treatment technologies is nitrooxidation in fluidized layer. It is a process of thermo-chemical treatment that consists of surface nitridation with following oxidation. Thanks to this process the corrosion resistance or mechanical properties are significantly increased [1,2,3,4]. In manufacture process it is important not just to produce high quality surface treated materials, but also to effectively join them. That is why there is an attention focused on research of new and modified welding technologies. Cold metal transfer process is one of the newer welding technologies, especially in combination with laser beam [5,6]. 1.1 Material properties Low-carbon deep-drawing steel DC 01 EN 10130/91 of 1 mm thickness was used for experiment. Chemical composition of steel as well as basic mechanical properties are provided in Table 1 and Table 2. 1

2 Table 1 Chemical composition of DC 01 EN 10130/91 EN marking C Mn P S Si Al [max. %] [max. %] [max. %] [max. %] [max. %] [min. %] DC 01 max max max max Table 2 Mechanical properties of DC 01 EN 10130/91 R e R m A 80 [MPa] [MPa] [%] Thermo-chemical treatment Nitrooxidation process was carried out in Kaliare a.s., Pova ská Bystrica, Slovakia. The nitridation fluid environment consisted of Al 2 O 3 grains of 120 m in diameter wafted by gaseous ammonia. After nitridation the oxidation process started immediately. Oxidation itself was carried out in a vapours of distilled water, supplied to the furnace chamber. Processes parameters are shown in Table 3. Table 3 Nitrooxidation process parameters Nitridation temperature Nitridation time Oxidation temperature Oxidation time [ C] [min.] [ C] [min.] Experimental The experiment was carried out in First Welding, Inc., Bratislava, Slovakia. Together fifteen specimens were prepared with different parameters. Welding equipment consisted of laser and CMT machine. Geometrical configuration of CMT torch in relation to laser welding head is shown in Fig. 1. CO 2 laser took a part as first in welding process in order to destroy the surface layer and to create the weld bath. CMT torch followed the laser head direction in constant distance. The parameters of both processes are provided in Table 4. CuSi3 solder was used as a filler in CMT joining process. Fig. 1 Geometrical configuration of laser head and CMT torch in hybrid soldering, bent angle of CMT nozzle = 50, angle of laser head roll = 30, b laser beam from CMT wire distance in point of interaction with base metal 2

3 Table 4 Parameters of joining CMT Laser Current [A] Voltage [V] Wire feed [m.min -1 ] Velocity [mm.s -1 ] Shielding gas Power [kw] Shielding gas Argon 4 Helium Argon 4 Argon Argon 4 Argon Argon 4 Argon Argon 4 Argon Argon 3 Argon Argon 3 Argon Argon 4 Argon Argon 3 Argon Argon 3 Argon Argon 3 Argon Argon 3 Argon Argon 2 Argon Argon 2 Argon Argon 4 Argon 30 Specimen No. Velocity [mm.s -1 ] 2 EXPERIMENT RESULTS Visual inspection was used as a first step of quality evaluation. In further evaluation macroscopic and microscopic analysis as well as micro hardness measurement, semi-quantitative chemical analysis and tensile strength test were carried out. Visual inspection showed some defects, mainly irregular shape and reinforcement of weld bead. The best specimens were selected for further analysis. Results of macroscopic analysis are provided in Fig. 2. High porosity was observed in almost every specimen during the macrostructure analyses in full length of the joint. There were also observed irregularities of weld joint shape. Probable reason of the high porosity lies in surface treatment due to the high content of nitrogen and oxygen. Until now it is just hypothesis as it has not been confirmed yet and is subject of further research. a) b) c) Fig. 2 Macrostructures of joints a) Specimen No. 2 b) Specimen No. 3 c) Specimen No. 1 Fig. 3 documents microstructure of solder HTHAZ (High Temperature Heat Affected Zone) transition. Presence of HTHAZ is caused mainly by heat input of the laser beam, where in combination of brazing process temperature the material got overheated resulting into grain coarsening. 3

4 coarse acicular ferrite solder HTHAZ acicular ferrite polygonal ferrite Fig. 3 Microstructure of solder HTHAZ transition Microhardness measurement was performed on Buehler IndentaMet 1100 Series equipment with load of F = 981 mn (m = 100 g) and load time of 10 s. Figure 4 illustrates microhardness in area from the solder to the base material. High values of microhardness were observed in the solder area and even higher in area of HTHAZ interface. The highest values of microhardness were observed in this interface. EDAX analysis (JEOL JSM-7600F) was performed to determine the reason of microhardness increase at solder HTHAZ interface. Fig. 4 Microhardness values across the joint (Specimen No. 3) Analysed areas of specimen No. 3 can be seen in Fig. 5a. Fig. 5b shows the copper and iron content in the solder. Results proved high concentration of copper (spectrum No. 1) in certain areas that corresponds with the solder chemical composition. There are also present areas rich in iron at the expense of copper concentration (spectrum 2 and 3). Especially areas with high concentration of iron (Fig. 5) caused increase of solder hardness. 4

5 Cu Fe a) b) Fig. 5 Measuring areas of EDAX analysis (a) and a surface distribution of copper and iron (b) in the solder Table 5 Results of solder EDAX analysis Spectrum O Si Mn Fe Cu [weight. %] [ weight. %] [ weight. %] [ weight. %] [ weight. %] Tensile strength test was done on specimens with dimensions of 25 x 250 mm on EU 40 equipment. The test result is shown in Fig. 6a. It is obvious that the crack appeared almost immediately after applying the load in joint axis. Expected reason is a high porosity of the joint confirmed also by fracture area appearance (Fig. 6b). Force to the joint failure was 7.5 kn. Fig. 6 Results of tensile test a) Specimen No. 2 after test b) fracture area 3 CONCLUSION In experimental activity the CMT laser hybrid brazing technology was applied for metallurgical joining of steel sheets treated by nitrooxidation. This technology was chosen due to its advantages, such as lack of spatter and limited heat input. The role of laser beam application was to destroy surface layer being responsible for increased porosity during arc welding process. The results confirmed that this technology is not suitable for joining this way treated steel sheets. There was observed irregular and reinforced weld bead together with excessive amount of porosity responsible for unsatisfactory results of tensile test. EDAX analysis proved change in solder chemical composition during joining process resulting into hardness increase in solder and solder HTHAZ transition. Nevertheless, research activity is still in progress in order to find appropriate joining parameters for reducing porosity of specimens and to obtain a joint with required quality. 5

6 This paper was prepared under the support of APVV grant. LITERATURE REFERENCES [1] MARÔNEK, M: CMT Welding of Steel Sheets Treated by Nitrooxidation. In: Proceedings of the International Conference on Advances in Welding Science & Technology for Construction, Energy & Transportation ISBN [2] BÁRTA, J.: Welding of Special Treated Thin Steel Sheets. Disertation Thesis [3] MARÔNEK, M.: Comparison of laser and electron beam welding of steel sheets treated by nitrooxidation. In: Congresso da ABM (CD-ROM). - ISSN X [4] MICHALEC, I.: CMT Technology Exploitation for Welding of Steel Sheets Treated by Nitrooxidation. Diploma Thesis [5] MARÔNEK, M., LAZAR, R., DOMÁNKOVÁ, M.: Low Carbon steel sheets treated by nitrooxidation in fluidized bed. In Strojárstvo - Strojírenství, 2007, vol. XI, No. 4, p [6] Available on the web site: <: 6